你提到“上述黑体部分”,我没有找到。我估计你大概指的是its alliance with the public playhouses established吧?我读完整个句子后,我的语感是:its alliance with the public playhouses established, 是省略句=its alliance with the public playhouses (had been) established, 因此我不认为它属于独立主格结构。这里应该是按先后发生的顺序进行描述的。A native literary drama had been created,然后its alliance with the public playhouses had been established, 最后some of its great traditions had been begun. 因此,我觉得此句不适合用独立主格结构。
个人理解,仅供参考!